One source of efficiency loss in high purity O.sub.2 plants with byproduct argon is the nitrogen stripping section of the N.sub.2 removal column. The N.sub.2 stripping section is above the argon stripping section and below the feed point; the withdrawal point of the crude oxygen containing argon is between the argon and N.sub.2 stripping sections. In most prior art flowsheets, both conventional dual pressure and low energy triple pressure, this section has more reboil than necessary, resulting in large mixing losses and decreased argon recovery. The minimum reboil required up the N.sub.2 stripping section, i.e., the amount necessary to avoid "pinching out", in the absence of an intermediate reboiler, is determined by the composition and quality of the column feed. The column feed is usually the HP rectifier liquid bottom product, conventionally known as "kettle liquid", of about 34 to 38% oxygen composition. Kettle liquid is usually evaporated at the overhead of the argon rectifying section to reflux the argon rectifier; thus, part of the N.sub.2 removal column feed is fully evaporated kettle liquid, of about 34 to 38% O.sub.2 composition. This establishes a minimum V/L (molar vapor flow divided by molar liquid flow) in the N.sub.2 stripping section of about 0.6, corresponding to 30.6 moles of vapor ascending and 51 moles of liquid descending, all per 100 moles of air feed.
Typical operating conditions for the conventional dual pressure cryogenic high purity oxygen flowsheet with argon sidearm (rectifier) are disclosed by M. Streich and J. Dworschak in the technical article "Production of Large Quantities of Oxygen by an Improved Two-Column Process", appearing at pages 516-517 of the Proceedings of the XV International Congress of Refrigeration, 1979.
It is possible to reflux the overhead of the argon rectifier by latent heat exchange with intermediate liquid from the N.sub.2 stripping section, instead of evaporating kettle liquid. This is disclosed in U.S. Pat. No. 2,316,056. If an intermediate height of the N.sub.2 stripping section is selected where the vapor O.sub.2 composition is appreciably greater than 34 to 38%, e.g., about 41% or higher, then the minimum V/L in the N.sub.2 stripping section can be significantly decreased to 0.54 or lower (a 10% reduction) and the reboil up the argon rectifier correspondingly increased. This will increase argon recovery. However, it has the following disadvantage: in order to achieve the desired purity of the crude argon, on the order of 95%, it is necessary that the argon rectifier have substantially more theoretical stages of countercurrent vaporliquid contact, for example 40 as compared to 20 in the N.sub.2 stripper. This places the argon rectifier overhead at a considerably different height than the appropriate intermediate height of the N.sub.2 stripping section. Thus, regardless of whether the reflux condenser is located at the argon rectifier overhead, or the N.sub.2 stripper intermediate height, or external to both columns, at least one reflux liquid pump will be required to compensate for the height difference.
U.S. Pat. No. 4,670,031 by the present applicant, which is incorporated by reference, discloses that in order to increase argon recovery it is necessary to send more reboil up the oxygen-argon rectifying section and correspondingly less reboil up the nitrogen-crude oxygen rectifying section. That application also discloses a means for both further increasing argon recovery and for avoiding the tray height disparity cited above which necessitates a pump. The disclosed means is to exchange latent heat from intermediate height argon rectifier vapor to intermediate height N.sub.2 stripper liquid. Since the intermediate argon rectifier vapor is at a higher temperature than the overhead vapor, it can provide intermediate reboil to a lower (warmer) height of the N.sub.2 stripper, i.e., a height corresponding to even higher O.sub.2 composition. This further reduces the fraction of reboil required up the lower part of the N.sub.2 stripper, and correspondingly increases the reboil possible up the lower section of the argon rectifier, thus increasing argon recovery. Also, it is possible to locate the intermediate height of the argon rectifier such that liquid return from the intermediate reboiler/intermediate reflux condenser is by gravity, avoiding the need for a pump.
The disadvantages of this configuration are that an additional heat exchanger is required; and that the reboil up the top half of the argon rectifier is low, where the relative volatility is also very low.
The same advantages from exchanging latent heat from an intermediate height of the argon rectifier to an intermediate height of the N.sub.2 stripping section are also obtainable in low energy triple pressure flowsheets, as disclosed in U.S. Pat. Nos. 4,578,095 and 4,605,427.
A second source of efficiency loss in dual pressure high purity oxygen plants is the large .DELTA.T of the argon rectifier reflux condenser, on the order of 4.degree. to 5.degree. C. This is the difference between crude argon condensing temperature and kettle liquid evaporating temperature.
It is known to evaporate kettle liquid at a pressure appreciably above the N.sub.2 rejection column pressure, by exchanging latent heat with HP rectifier overhead vapor, and then expand the vapor to column pressure. Examples are presented in the Streich and Dworschak article cited above, and in U.S. Pat. No. 2,753,698. Since this technique results in appreciable vapor flow bypassing the argon stripper, it is not appropriate for the production of high purity oxygen.
It is also known to evaporate kettle liquid at essentially the same pressure as the N.sub.2 removal column by latent heat exchange with HP rectifier vapor. This can be done via a single stage of evaporation (U.S. Pat. Nos. 4,208,199 and 4,254,629) by multiple stages of evaporation (U.S. Pat. No. 2,812,645). These flowsheets similarly are not suited for production of large quantities of high purity oxygen plus byproduct argon.
Copending application No. 853461 filed 4/18/86 by the present applicant discloses means to increase O.sub.2 delivery pressure while retaining high recovery in high purity O.sub.2 plants by warm companding a minor fraction of supply air to above supply pressure, totally condensing it to evaporate product oxygen, and splitting the liquid air as intermediate reflux to both the HP rectifier and N.sub.2 removal column.
U.S. Pat. No. 4,072,023 discloses means for increasing O.sub.2 production pressure by cold companding the gaseous O.sub.2 product stream using extra expansion power not necessary for process refrigeration.
What is needed, and one objective of this invention, is to achieve increased argon recovery in a high purity O.sub.2 flowsheet without incurring at least some of the disadvantages present in prior art flowsheets: need for pumping reflux liquid uphill, need to provide an additional heat exchanger, or need to reduce reboil in top half of the argon rectifier. A further objective is to recover useful energy in place of the inefficient large .DELTA.T heat exchange occurring in conventional argon rectifier reflux condensers. A most preferred solution would satisfy both of these objectives (solve both problems) simultaneously.